δ13C values of soil organic carbon and their responses to C3 and C4 plants shift in Mengzi karst graben basin, SW China

Abstract

Understanding the controlling factors of soil organic carbon isotope (δ¹³C_SOC) change and the vegetation succession process is crucial to guide ecological restoration and agricultural cultivation in karst rocky desertification region. However, the information about the combination of C₃ and C₄ plant distribution and rocky desertification remains unknown. Soils from different landforms, including basin, slope, and plateau, were sampled to investigate the spatial variance of the δ¹³C_SOC distribution characteristics. The contribution of C₃ and C₄ plant species for δ¹³C_SOC under the different rocky desertification grades (LRD: light rocky desertification; MRD: moderate rocky desertification; and SRD: severe rocky desertification) in Mengzi karst graben basin of Southwest (SW) China was also discussed. The δ¹³C_SOC  value decreased with the increase of altitude from basin, slope to plateau. At the same landform, different rocky desertification grades had no significant effect on the δ¹³C_SOC in slope and plateau. Nevertheless, there were significant differences of δ¹³C_SOC C between LRD and SRD in the basin. The C₄ plants account for more than 70% in the basin and slope, while C₃ plants account for more than 70% in the plateau. This may be due to the long-term cultivation of corn in the historical period in the basin and slope. However, the plateau area is not suitable for the growth of C₄ plants such as corn due to the cold climate. In addition, in the same landform, with the aggravation of rocky desertification, the proportion of C₄ plants for δ¹³C_SOC increased with the proportion of C₃ plants decreased. With the aggravation of rocky desertification, the composition of vegetation species changed from arbour (C₃ plants) to small shrubs and herbs (C₄ plants).